Cancer is a disease of uncontrolled cell growth causing local expansion of a tumor and, potentially, distant metastases. One mechanism by which cancer cells grow is by avoidance of apoptosis, or programmed cell death. Alterations in apoptotic pathways have been linked to cancer cells being resistant to standard treatments, e.g., chemotherapeutics or radiation, and to the incidence and progression of cancer. See, e.g., E. Dean et al., “X-linked inhibitor of apoptosis protein as a therapeutic target,” Expert Opin. Ther. Targets (2007) 11(11):1459-1471
The two basic pathways for apoptotic cell death are the intrinsic pathway and the extrinsic pathway. The intrinsic apoptotic pathway can be initiated by various mechanisms including cellular stress and drug-induced DNA damage. The extrinsic pathway can be initiated by activation of the death receptors by a chemokine. Initiation of either pathway results in the activation of a family of proteases called caspases. Once activated, the caspases can act to cleave a variety of substrates creating a cascade of events that lead to the activation of the effector caspases 3 and 7 and eventual cell death. The IAP family of proteins can bind to and inhibit the activity of caspases thus inhibiting apoptosis. See, e.g., Dean, supra at 1460.
The IAPs can contain up to three copies of homologous structural domains called baculoviral IAP repeat (BIR) domains, BIR1, BIR2 and BIR3. The BIR3 domain of the prototypical IAPs, cIAP and XIAP, can bind to and inhibit activated caspase 9. The BIR2 domain, in contrast, binds to and inhibits caspases 3 and 7. The proapoptotic protein Smac (also known as DIABLO) can block the BIR2 and BIR3 domains of IAPs competing with activated caspases resulting in release of the activated caspases from the IAPs and completion of the apoptotic program. See, e.g., S. Wang, “Design of Small-Molecule Smac Mimetics as IAP Antagonists,” Current Topics in Microbiology and Immunology 348, DOI 10.100782_2010_111, pp. 89-113.
Peptides and small molecules have been reported to bind to the BIR3 region of XIAP and cIAP, mimicking the action of Smac protein and releasing activated caspases. See, e.g., Dean, supra; and M. Gyrd-Hanse et al., “IAPs: From caspase inhibitors to modulators of NF-κB, inflammation and cancer,” Nature Review/Cancer, August 2010, Vol 10:561-574.